There can be many reasons for jealousy. Some people are inherently jealous while some are not...Depends on the person.
The major reason for this can be presence of another woman/ man in your spouse's life. Apart from this can be trust issues, financial and other insecurities. Jealousy can take any form, that does not involve only breach of trust. It can be if you are insecure of your partner's looks. Also it can be if your spouse earns more than you. The solution to this to be aware (of your own shortcomings) see the situation for what it is, check your ego...That's the best way to handle this relationship problem.
In relationships people tend to be very possessive of their partners because they don't want to lose what gives them the sense of security and joy that they get from them, and see it as a special and exclusive bond between them. When there is any threat to this sense of emotional security that they have, they might get jealous and act in irrational ways. For example, if an attractive person talks and possibly flirts with the partner, the fear of possibly losing that partner to them activates, which in turn triggers the jealousy.
When we are in a romantic relationship, we often like to think that our partner is exclusive to us because they have given joy and love that seems special. When there are other people in the picture who seem like threats, we start to feel jealous and possessive of them. This is because we are afraid that the threat will take away our special love and leave us with nothing.
Jealousy comes from a place inside us that feels inadequate for who we are or what we have (or don’t have). When a person becomes jealous over a spouse’s behavior with another person of the opposite sex, it can mean that the spouse was inappropriately flirting, which makes their partner feel insecure. But it could also be that the person already felt insecure, and seeing their spouse with someone else triggers a feeling that was already there. Either way, the only way to address the jealousy is to heal the feelings of inadequacy and communicate the problem to the spouse.